The president of the National Farmers Union accused the big supermarkets today of "bullyboy tactics" by trying to renegotiate contracts and reduce prices.

Peter Kendall said the supermarkets were trying to pre-empt a code of practice designed to give farmers and suppliers increased protection from such abuses of power. The groceries supply code of practice (GSCOP) comes into force tomorrow.

Speaking at the Soil Association's annual conference in Birmingham, Kendall said: "Recent reports to me about retailer behaviour in the supply chain have demonstrated nothing but bullyboy tactics.

"This week should have been marked by a sea change in grocery supply chain relationships with our farmer and grower members but instead in the past 10 days I have heard how suppliers to major retailers have faced some of the most unreasonable demands for retrospective payments and changes to trading terms that we've ever seen."

Kendall said some farms had been forced out of business, while many had decided to go back to being smaller operations . "It is very unfair on these farmers as farming is all about the long term in terms of investment etc. This is something that the current and future government must look at very carefully."

The NFU was instrumental in getting the government to agree to appoint a supermarkets ombudsman, but that has now been delayed until after the election.

Stephen Robertson, director general of the British Retail Consortium, said: "The effort and money retailers are putting into demonstrating they meet the GCOP's requirements and informing suppliers shows they accept their responsibilities as major players in the supply chain."

He added: "Retailers want successful, sustainable long-term relationships with suppliers. How else would they get the quantity and quality of goods their customers want?"

At the two-day conference, which opened today, organic and conventional farmers were urged to put up a united front against the power of supermarkets and international food companies. In a panel discussion about the challenges for farmers of feeding a growing population, some delegates warned that farming was struggling to recruit workers as it did not have an attractive image.

Rebecca Hoskings, a Devon-based organic farmer and campaigner who led a ban on plastic carrier bags in her home town of Modbury, said neighbours had questioned her decision to switch from a TV and film career to take over her father's livestock farm.

"You're seen as the lowest of the low if you go into farming," she said.

Earlier, Patrick Holden, the Soil Association's policy director, denied that organic food and farming was for a privileged few. In his opening address he said: "The paradox is, there's this view that organic is elite, it's expensive, it's a lifestyle choice for people who can afford it. But as far as I'm concerned it's not elitist to believe that everyone should have the right to high-quality, nutritious food from sustainable farming systems. We need to reach out to society."

The Liberal Democrats' farming spokesman, Roger Williams, told delegates: "Organic farmers and conventional farmers both need to be less defensive if we are going to learn the lessons that each system can offer. We need to listen to each other if we are to produce as much food as possible without destroying the world's resources."

Earlier, the food and farming secretary, Hilary Benn, told the conference that the organic farming movement could pass on invaluable skills and knowledge to conventional competitors.

He said: "The world will turn to you in the years ahead to ask how to be less dependent on oil-based fertilisers because of your expertise. The skill, determination and will of this movement are the skills we will need if we are going to achieve change."